Power System Stability Issues and Remedial Action System Stability... · 2019. 1. 11. · Power System Reliability The North American Electric Reliability Corporation (NERC) defines

By: Chandan Kumar, Deputy Manager

ERLDC,POSOCO

Power System Stability Issues and

Remedial Action

“Electricity is really just organized lightning”

-George Carlin

“When the laws of Economics and Physics collide, Physics always wins”

-George C. Loehr

Power System Reliability

Reliability

SecurityAdequacy

• Adequacy relates to the existence of sufficient facilities within the system to satisfy the consumer load

demand at all times.

• Security relates to the ability to withstand sudden disturbances

Reliability of a power system refers to the probability of its satisfactory operation over the long run. It denotes theability to supply adequate electric service on a nearly continuous basis, with few interruptions over an extended timeperiod

- IEEE Paper on Terms & Definitions, 2004

11-Jan-19 ERLDC, POSOCO 3

Power System Reliability

The North American Electric Reliability Corporation (NERC) defines two components ofsystem reliability:

• Adequacy – Having sufficient resources to provide customers with a continuous supplyof electricity at the proper voltage and frequency, virtually all of the time. “Resources”refers to a combination of electricity generating and transmission facilities, whichproduce and deliver electricity; and “demand-response” programs, which reducecustomer demand for electricity.

• Security – The ability of the bulk power system to withstand sudden, unexpecteddisturbances such as short circuits, or unanticipated loss of system elements due tonatural or man-made causes.

Power system is operated in a reliable and secure manner so that system stability is not endangered.

Contd….


Power System Stability

Angle stability Voltage stability

Small signal stability Transient stability Large disturbance Small disturbance

Mid term Long term

Study period up to 10 secs

Study period up to several minutes

Study period up to tens of minutes

Power System Stability

Frequency stability


• Maintaining Frequency Closer to Nominal Frequency (50 Hz)

1. Frequency Stability

50 Hz

Generation Load


To arrest change in frequency, control actions are required

Frequency Control can be divided into three overlapping windows of time

Primary Frequency Control Control provided by Interconnection

Secondary Frequency Control (AGC) Control provided by individual control area

Tertiary Frequency Control Control provided by individual control area in coordination with other control areas

Frequency Control


Frequency Control Actions

df/dt, UFLS/SP

S

Depletion of Network/Tripping

AGC

Generator

Governor

System Operator

Power System

Emergency Control/

Defensive Mechanism

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Demand side controlGeneration side control

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Source: Royal Institute, KTH, EMS


• Keeping Adequate Synchronizing and Damping Torque in system to

reach new equilibrium point after any small/Large disturbance.

• Lack of Synchronizing Torque : The disturbance on the system is quite severe

and sudden and the machine is unable to maintain synchronism under the

impact of this disturbance.

• Lack of Damping Torque : For inadequate amount of damping torque, the

rotor angle undergoes oscillations with increasing amplitude.

2. Angular Stability



Source : https://nptel.ac.in/courses/108107028/module6/lecture1/lecture1.pdf

• Delayed Fault Clearance Near to CGPL Power Plant.

• Tc > Critical Clearing Time for CGPL Units• Generator not able to Push Active power

to System due to Fault and overspeed/accelerate• Leading to pole slip and out of step

protection operation.• Centre of swing lies in CGPL causing

instability.• All Outgoing lines tripped on Power

Swing Protection.

Lack of Synchronizing Torque : Pole Slip/Out of Step


Fig: CGPL Voltage from Simulation


• Ensuring adequate number of lines for full evacuation of power.

• Ensuring Angular separation between adjacent nodes are within limit.

• Checking Stability under various criteria through simulation.

• Using Fast Acting System protection Scheme to reduce theaccelerating power available in the system to maintainsynchronization.

• Finding Critical clearing time (tc) for various operating scenario andhaving suitable remedial scheme.

• Improving voltage profile by dynamic Reactive support under largedisturbance.

• Reducing Angular difference in case of contingencies by reducing load/generation.

Remedial Action


Lack of Damping Torque : Low Frequency Oscillation


• Ensuring adequate number of lines for full evacuation of power.

• Ensuring Angular separation between adjacent nodes are withinlimit.

• PSS to be properly tuned and verified with tests.

• Using Fast Acting System protection Scheme to reducegeneration and grid stress.

• Tuning of HVDC/FACTS Power Oscillation Damping (POD)Controller.

Remedial Action


• Maintain steady acceptable voltages at all buses in the system

• System enters a state of voltage instability during a disturbanceor increase in load demand

• Reason : Inability of a power system to meet the demand forreactive power.

• Remedial Action :

• Ensure Voltage Stability study for Load rich areas

• Adequate Reactive Support Margin availability

• Ensuring N-1 and N-1-1 Reliability criteria

• Implement UVLS Scheme

3. Voltage Stability



• Voltage Collapse at Agartala dueto loss of reactive Support from400 kV System.

• Voltage dependent load gotstalled in Tripura Power Systemand Bangladesh (North Camila)

Voltage Dip During Fault And FIDVR• Induction Motor : Power Requirement depend on Frequency and Voltage.

• Air conditioning Load and Industrial Motor Load : Either Stall (In absence or protection) or Trip

(Thermal/Under Voltage/LVRT Protection) during voltage dip

• They will start quickly after Stall and trip and draws huge reactive power.

• Fault induced Load Loss (FILL) : A Major Portion of Load Can trip or Stall during fault and come

back slowly after fault clearance

• If the LVRT protection is not there and large amount of load stall under fault : After fault

clearance, it requires large reactive power from the system to run (Its Induction Motor)

• Fault Induced Delayed Voltage Recovery (FIDVR) : Large reactive power is drawn, Voltage

remains low for large duration and additional load trip specially which have LVRT.


3 Phase Symmetrical 220 kV Bus Fault near Industrial Load

Centres In Maharashtra.

Delayed Fault clearance (Voltage Reduced by 50 %)

Around 1000 MW load was lost.

No report of loss in terms of lines/ICT tripping at

distribution/transmission level.

Key Learning :

• No Tripping is observed yet frequency increased

indicating large quantum of load loss.

• Load Loss can occur when fault is severe and delayed

clearance is there. (Not FIDVR Case)

• Surprise for System Operator.

Case Study 1 : Padghe Event

Voltage

Demand and Frequency


• Summer Season : High Concentration of Air

condition load in Delhi and nearby area.

• Phase to Phase Fault on 400 kV Transmission

line near Delhi and delayed fault clearance.

• Fault clearance is delayed.

• 3700 MW load was lost based on SCADA and

PMU data. High Shoot up in frequency.

• No report of loss in terms of lines/ICT tripping

at distribution level.

• Slow Voltage recovery and FIDVR Characteristic

Observed.

Case Study 2 : 400 kV Bawana – Mundka - II tripping event (FIDVR)

Voltage


Angular Separation in Grid and frequency Increased Drastically

Northern Region Demand Reduced by Large Quantum


FIDVR or Severe Fault Causing Load Loss becoming frequent and act as a challenge for System

Operator.

Characteristic of Such Events

• Any phase to phase or three phase of fault at 220 kV and above level near to load centers :

• Severe nature

• Cause load loss due to tripping of induction motor load on overcurrent/under voltage protection.

• Delayed fault clearance makes it more severe as causes more load loss.

• FIDVR event

• Occur during the summer season when load are majorly of single phase AC units’ load.

• Occur during severe fault near to the load centers along with its delayed clearance from the system.

• Followed by a sharp rise in frequency due to load loss and quick recovery with 5-20 minutes.


• Treatment of Distorted Voltage : Erroneous Frequency Calculation can result inTripping of the Inverter (Solar Plant)

• Wind Related LVRT protection setting for DFIG based Turbine having crowbararrangement : Limitation with How many times LVRT will operate within aduration of time.

• Protection Coordination and Islanding

Some Key Issues with Renewable & Inverters


Thanks [email protected]


Defense Mechanism


1. Operational States of the Grid

Normal

• Equality Constraint : Load+Loss=Generation• Inequality Constraint : All Parameters within Safe limit


• Normal state :All system variables are within the normal operating range

• Alert state : Security level falls below a certain limit of adequacy because of a

Event

• Emergency state : Severe disturbance

• In Extremis : Cascading outages

• Restorative state



• Ultimate Objective is to not reach the IN EXTREMIS State of Operation.

• Each Level having its own defense mechanism for returning back to previous state.

• Normal state :

• Operational Standards : CEA Standard and IEGC

• Operational Ranges for System Parameters : CEA Standard and IEGC

• Operational Criteria : IEGC and Operational Procedure

• Alert state :

• Primary , Secondary and Tertiary Control

• Automatic Control from FACTS/HVDC devices

• Load Generation Redispatch

• Manual Load Shedding and Generation Revival

• Under Voltage Load Shedding Scheme



• Emergency State

• System Protection Scheme

• Load Trimming Scheme

• Generation Reduction Scheme

• In Extremis

• Under Frequency Load Shedding Scheme

• ROCOF based Load Shedding Scheme

• Islanding Scheme

• Unintended System Separation

• Restorative state

• Black Start and Island Synchronization



2. Need of Defense Mechanism

Power System Operate with following Contingencies in Real Time:

• N : No Element is Out

• N-1: One Important element is Out

• N-1-1 : One element is out followed by other element

• N-K: Multiple elements are out

Real Time Operators ensure that there is no additional insecure operationafter these outages/contingencies. All Electrical Parameters are within theirprescribed band.


However, System Can go from N state to N-K with Events/Contingencies

That’s why there is a need of Defense Mechanism

2. Need of Defense Mechanism


3. Defense Mechanism and Types

• Need of Some Mechanism to recover the system back to Normal State of Operation.

• Mechanism placed in Grid for Recovering System from Alert/Emergency/ In Extremis

State are called Defense Mechanism for Power System.

• Automated Mechanism

• Quick recovery (Response Time is in millisecond to few seconds)

• Fast, Logic Based, Measurement Based, Reliable.

• Action can be Local or Global depending on design.

• Act as last line of defense before system go for blackout or cascaded tripping.


Defense Mechanism Can be Classified as :

• Based on Type of Action : Generation, Load, HVDC Set Point, FACTS.

• Based on Area of Action : Local and Global

• Based on Parameter : Frequency Based, Voltage Based, Current Based, Active

and Reactive Flow Based

• Based on Time : Instantaneous, Time delayed

3. Defense Mechanism and Types


3.Defense Mechanism in Indian Power System

• Dynamic Control of HVDC/FACTS, PSS Of Generators

• System Protection Scheme (SPS)

• Automated Under Voltage Load Shedding (AUVLS)

• ICT Load trimming Scheme (LTS)

• Automated Under Frequency load Shedding (AUFLS) and ROCOF based Load

Shedding

• Islanding Scheme


4.a. Dynamic Control of HVDC/FACTS, PSS Of Generators

• HVDC/FACTS are having dynamic control which depending of criteria provide

response to reduce the impact of event.

• Frequency Controller of HVDC

• Sub-synchronous Torsional Interaction(SSTI) Controller of HVDC

• Power Oscillation Damping (POD) of HVDC/FACTS

• Dynamic Reactive Support from SVC/STATCOM


4.a. Dynamic Control of HVDC/FACTS, PSS Of GeneratorsTa

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4.a. Dynamic Control of HVDC/FACTS, PSS Of Generators

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150

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t/s-5 0 5 10 15 20 25 30 35

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4.b. Automatic Under Voltage Load Shedding Scheme

• Power systems today are much more susceptibleto voltage collapses as increasingly depend ongeneration sources that are located remotely fromload centers.

• UVLS relays are meant for shedding of load whenthe voltage is going below the operational lowerlimit in order to avoid voltage collapse.

• IEGC 5.2.t Mandates : All Users, CTU and STUs shallprovide adequate voltage control measuresthrough voltage relay as finalized by RPC, toprevent voltage collapse. and shall ensure itseffective application to prevent voltage collapse/cascade tripping.


Basic Design Criteria :

1. Designed to coordinate with protective devices and control schemes for momentaryvoltage dips, sustained faults, low voltages caused by stalled air conditioners, etc.

2. The time delay should be in seconds, not in cycles

3. Must be on Bus VTs and coordinate with ICT/Lines Load Trimming Scheme.

4. Voltage pick-up points to set reasonably higher than the “nose point” P-V nose curve.

5. Enough load shed to bring voltages to minimum operating voltage levels or higher

Example Setting for UVLS

In case of two stage operation

• Alarm: If Voltage < 375 kV ; Stage 1: If Voltage < 372 kV for 5 sec; Stage 2: If Voltage < 370 kV for 10 Sec

In case of Single stage operation

• If Voltage < 370 kV for 5 seconds

4.b. Automatic Under Voltage Load Shedding Scheme


4.c. Load trimming Scheme (LTS)

• These are also SPS which are local in nature and has designated purpose ofsaving elements getting overloaded due to contingencies.

• Implementation for Line and ICT to avoid overloading.

• Several Such LTS scheme are there in Various States across India.

• Their operation during contingencies helps in arresting the eventwidespread.

• Basically Designed using Overcurrent Feature of the Relays with PLCC asCommunication Media.

• Need of Coordination between various LTS is very much necessary in aninterconnected system (One LTS Impacts Others): On two occasionsKolhapur ICTs (3 Nos) and Karad ICTs (3 Nos) tripped One iCT trippingeven after the LTS operation due to Non-Coordination.


• Frequency should be within prescribed band of 49.9-50.05 Hz.

• However, Frequency will fall rapidly with System Islanding/Largegeneration complex loss.

• AUFLS and ROCOF (df/dt) based automated load shedding are ways toarrest gradual as well as fast fall in frequency in the system (Frequency fallArrester)• ROCOF based relay arrest the fast fall in frequency while AUFLS will restrict the

frequency above a certain limit.

• AUFLS is global phenomenon and load shedding occurs as per design in thecomplete synchronized grid While ROCOF based load shedding quantum carieswith nearness to Frequency related event source.

• Quantum of such Load connected with AUFLS and df/dt relay changes with systemsize and connectivity.

4.d. AUFLS and ROCOF Relays for Load Shedding


Last AUFLS Operation : Separation of NEW and SR Grid

on 24 May 2015 during High import by Southern Region.



CGPL Event 12 March 2014 UMPP Loss Largest Contingency in the System

ROCOF Operated in Gujarat (636 MW) and Maharashtra (334)


ROCOF Relay Operated.


49.9 Hz

ROCOF Relay operation in Gujarat on 18 Jan 2017• Cause : Momentary Cessation of Full HVDC Mundra

Power Order of 2500 MW to Zero and Auto-Restart to2500 MW in 10 ms interval causing Large swing in WRand NR (See Frequency Plot)

• Root Cause : Simultaneous DC line Fault (RareContingency)

• ROCOF operated in Gujarat System near Mundra : 486MW load Loss

• PMU indicated ROCOF above 0.4 Hz/Sec (Highest Valueof ROCOF)



• When The system is moving towards in Extremis and Blackout , its desirable tosplit system having load/generation balance in Island for faster restoration.

• It’s the Last Measure : Islanding should take place only when all other defenseplan have been allowed their full opportunity to bring back and maintain systemintegrity and still the health of integrated system is on path of deteoriationtowards failure.

• Large Number of Islanding Scheme is Not Suitable for Integrated Grid.

• Operational frequency band for pre-islanding defense mechanism and islandingfrequency band sufficiently apart : Help in taking care of continued fall offrequency during the interval between relay pick-up and breaker opening.

• IEGC 5.2.n

4.e. Islanding Scheme


• Islanding to Occur : When all other defense plan ( fault clearance, SPS action, UFLS and df/dtload shedding etc.) have been allowed their final operational opportunity and the system isstill on the path of deterioration towards collapse.

• Islanding not to Occur : If there is still a chance that a distressed system can be brought backto emergency condition (or alert condition or normal condition) with operation of themechanism planed for integrated and interconnected states of Grid.

• Frequency Band Setting for Islanding

• Devised for Part of system which have Load generation balance : Preserve those areas whichinherently are in a position to achieve load generation balance post islanding.

To be Reviewed after 2 Years

Ramakrishna Committee Recommendation



• Generally Implemented for only those sub-parts of network :• Connect to rest of grid in an electrically radial manner with only a few

interconnecting lines• Have their own load generation balance to a large extent, requiring comparatively

smaller exchanges with rest of the grid.• Features to be incorporated In Islanding Scheme

• Adequate automated mechanism for achieving load generation balance.• Frequency control of islanded subsystem: Sufficient number/capacity of generating

units in on free governor mode of operation in the island.• Load connection/disconnection should be possible remotely from the dispatch

centre of the islanded sub-system.• Health of Monitoring System for all equipment in island System should be good.



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Documents

Power System Stability Issues and Remedial Action System Stability... · 2019. 1. 11. · Power System Reliability The North American Electric Reliability Corporation (NERC) defines